To assess the effect of gestational age and labor on the interleukin-8 (IL-8) concentration in whole cord blood and serum, IL-8 levels were determined simultaneously in cord blood serum and lysate in 134 infants. Following the elimination of some of the samples due to exclusion criteria, the data for 99 uninfected infants (71 term and 28 preterm) and 9 infants with neonatal bacterial infection delivered either vaginally or by elective or emergency cesarean section were analyzed. The effects of labor and gestational age were tested by analysis of variance. IL-8 was not detectable in the serum of 25 infants, whereas IL-8 levels in whole blood were measurable in all of the samples. The median IL-8 conncentrations in whole cord blood lysate were 106 pg/ml (range, 20 to 415 pg/ml) in preterm infants and 176 pg/ml (range, 34 to 1,667 pg/ml) in term infants. In contrast to the IL-8 levels in serum, IL-8 levels in whole blood were reduced after ECS. Gestational age had no independent effect on the IL-8 concentrations in either serum or whole blood; these concentrations increased in infected infants after labor. We conclude that the neonatal proinflammatory response to labor stress was more evident in the concentrations of IL-8 in whole blood than in serum. The levels of IL-8 in whole-blood lysate reflect proinflammatory stimulation in neonates and may be a useful diagnostic tool for the early diagnosis of neonatal infection.Interleukin-8 (IL-8) belongs to the class of proinflammatory "CC" chemokines defined by the position of two cysteine groups and is synthesized predominantly by monocytes. Its active form effectively activates neutrophil granulocytes, advancing the chemotaxis and synthesis of myeloperoxidase, thus suggesting a critical role in host defense to infectious diseases (1, 2).The IL-8 concentration in serum has been studied as a diagnostic marker of neonatal bacterial infection (NBI) (10, 11) and has been shown to be an early marker of neonatal bacterial infection, whereas the concentration of C-reactive protein (CRP) increases after 12 to 24 h in the course of systemic infectious disease. A "diagnostic gap " exists between the decline of IL-8 after 4 to 6 h and the increase in C-reactive protein at 12 to 24 h, which is a well-established marker of confirmed bacterial infection. Due to the rapid serum clearing of IL-8, its value as a monitoring parameter of infectious disease may be limited. In vivo, a large proportion of IL-8 is associated with erythrocytes and leukocytes (polymorphonuclear leukocytes and peripheral blood mononuclear cells), and the concentration in serum represents only a small fraction of the total amount of IL-8. The measurement of cell-associated IL-8 reflects more quantitatively its production and adds information regarding the stage of the disease and the patient's inflammatory response (7,9,13,14). Total IL-8 can be measured in whole blood after cell lysis with a good analytical test (16). In healthy adults, we previously observed an IL-8 peak level of Ͻ12 pg/ml with a median of 83 p...